Today, radio base stations, (RBS) of mobile or radio telecommunication systems usually share the same antenna of an antenna system in order to minimize the visual impact on the site. This is also known as co-siting. The radio base stations may belong to different radio access networks such as the GSM system (Global System for Mobile Communications) and/or the wide band code division multiplexing (WCDMA) system. In addition to antenna-sharing, feeder-sharing has become more and more popular as it offers the possibility to reduce the number of cables and the number of electronics in the site. It is also possible to combine feeder sharing and antenna sharing when co-siting different access networks operating at different frequency bands e.g. GSM 1800 MHz and WCDMA 2100 MHz or different access networks operating at different frequencies within the same band, e.g. GSM 1900 MHz, WCDMA 1900 MHz. Usually, a filter combiner capable of both combining and separating signals, is connected to the co-sited radio base stations to enable feeder-sharing between two or several systems. A filter combiner is a radio frequency component that acts both as a duplex filter and/or as a diplex filter. The duplex filter may for example combine the transmit (TX) and receive (RX) signals of a specific frequency band into a common signal, whereas the diplex filter combines combined TX/RX signals from separate frequency bands into a common signal—for example, the TX/RX signals of GSM 900 MHz and 1800 MHz can be combined on a shared antenna feeder cable and split into separate signals at the top of the antenna mast. The diplex filter and the duplex filter are also capable in separating signals. An example of a traditional antenna system wherein two co-siting radio base stations share the same antenna and also share the same feeder (or filter combiner) is discussed below with respect to FIG. 1.
Referring to FIG. 1 (Prior art), there is shown a block diagram of an antenna system 100 described in the international patent application WO 2006/121402. The antenna system 100 comprises two co-sited radio base stations 40 (RBS A) and 50 (RBS B) that are both connected to a filter combiner 10. The filter combiner 10 is further connected to an antenna 60. The radio base station 40 (RBS A) belongs to a first radio access network, e.g. GSM or WCDMA whereas the second radio base station 50 (RBS B) belongs to a second radio access network, e.g. WCDMA or GSM. As illustrated in FIG. 1. the antenna 60 is adapted to receive a signal RX which is a combination of a first receive signal RX1 and a second receive signal RX2. The first signal RX1 corresponds to a signal of a first radio access network, whereas the second signal RX2 corresponds to a signal of a second radio access network. The antenna 60 is also capable in transmitting a transmit signal TX being a combination of a first transmit signal TX1 and a second transmit signal TX2. The signal TX1 corresponds to a signal of a first radio access network whereas the signal TX2 corresponds to a signal of a second radio access network. As mentioned earlier, the filter combiner 10 is capable in separating the receive signal RX and the transmit signal TX. The filter combiner 10 also combines signals TX1 and TX2 prior to feeding the antenna 60. When a receive signal RX is received by the filter combiner 10 via antenna 60, the filter combiner 10 first feeds the first radio base station 40 (RBS A) with the RX signal. The radio base station 40 (RBS B) subsequently separates the RX signal into the RX1 and RX2 signals and feeds the second base station 50 (RBS B) with the RX2 signal via a port P8 of the filter combiner 10. The first radio base station 40 (RBS A) operates therefore as a master radio base station whereas the second radio base station 50 (RBS B) operates as a slave radio base station. This is because the filter combiner 10 is composed of two narrow band transmit filters, labelled 10c and 10d, which prevent the wide band RX signal to pass through the TX2/RX2 path. The narrow band is on the other hand tuneable in the wide band. It should be noted that the narrow and wide bands discussed above correspond to frequency bands or radio channels allocated to the systems in the site. As an example, two frequency bands, typically of 75 MHz each, are allocated to the GSM 1800 MHz system, whereas radio channels that are 5 MHz wide are allocated to the WCDMA system.
Referring back to FIG. 1, the radio base station 40 (RBS A) is, according to this prior art, forced to operate as a master radio base station because it must feed the slave radio base station 50 (RBS B) with the RX2 signal. The RX2 signal is prevented from passing any of the two narrow band filters 10c and/or 10d. 
Tests performed at the site showed that in order to achieve as good performance as possible and to better deal with sensitivity issues of the co-siting systems, it is preferable to use the WCDMA radio base station as the master and the GSM radio base station as the slave. Therefore the radio base station 40 (RBS A) is, according to prior art, a WCDMA master radio base station. A drawback with this solution is that by using the WCDMA radio base station as the master base station, the spectrum utilization is greatly reduced because, as mentioned above, the WCDMA system only requires approximately 5 MHz of bandwidth.
An additional problem with the co-siting solution described above is that the radio base station 40 (RBS A) is forced to operate as a master, i.e. an operator can not use RBS B as a master by simply connecting the RF path TX2/RX2 to RBS A and the RF path TX1/RX to RBS B since this will reduce the operational performance of the site. In other words, simple switching of the RF paths may increase the interference between the two systems, and may also increase the unwanted emission generated from one system to the other. A solution to this problem would be to perform a complete new reconfiguration of the site, which most probably is an unfeasible solution to the operator of the site.
Yet another problem with the co-siting solution described above concerns the case where the operation of the master base station 40 (RBS A) is, for some reasons, interrupted. In such a case, the slave radio base station 50 (RBS B) is also affected since it can only transmit signals via the antenna 60 but is unable to receive signals until the master radio base station 40 (RBS B) is repaired or replaced.